Jan Oort began studying stellar dynamics under Jacobus C. Kapteyn at the University of Groningen, where he completed his Ph.D. under Pieter van Rhijn in 1926. While a graduate student he spent two years at Yale University with Frank Schlesinger, after which he worked at the University of Leiden from 1924 until his death. In 1927 Oort confirmed Bertil Lindblad’s hypothesis of galactic rotation by analyzing motions of distant stars. Oort found evidence for differential rotation and founded the mathematical theory of galactic structure. During World War II Oort started Hendrik C. van de Hulst on the successful search for a radio spectral line. Following the war Oort led the Dutch group which built a 25-m radio telescope at Dwingeloo and used the 21-cm line to map hydrogen gas in the Galaxy. They found the large-scale spiral structure, the galactic center, and gas cloud motions. In 1950 Oort proposed the now generally accepted model for the origin of comets. He later showed that light from the Crab Nebula is polarized, confirming Iosif S. Shklovskii’s suggestion of synchrotron radiation. He continued researching the Milky Way and other galaxies and their distribution until shortly before his death at 92. Oort was a leader in European astronomy and played a major role in the rise of the European Southern Observatory and other international organizations.